Joining Threads in Java

java.lang.Thread class provides the join() method which allows one thread to wait until another thread completes its execution. If t is a Thread object whose thread is currently executing, then t.join(); it causes the current thread to pause its execution until thread it join completes its execution.
If there are multiple threads calling the join() methods that means overloading on join allows the programmer to specify a waiting period. However, as with sleep, join is dependent on the OS for timing, so you should not assume that join will wait exactly as long as you specify.
There are three overloaded join functions.

join(): It will put the current thread on wait until the thread on which it is called is dead. If thread is interrupted then it will throw InterruptedException.Syntax:

public final void join()

join(long millis) :It will put the current thread on wait until the thread on which it is called is dead or wait for specified time (milliseconds).Syntax:

public final synchronized void join(long millis)

join(long millis, int nanos): It will put the current thread on wait until the thread on which it is called is dead or wait for specified time (milliseconds + nanos).Syntax:

public final synchronized void join(long millis, int nanos)

// Java program to explain the

// concept of joining a thread.

importjava.io.*;

// Creating thread by creating the

// objects of that class

classThreadJoining extendsThread

{

@Override

publicvoidrun()

{

for(inti = 0; i < 2; i++)

{

try

{

Thread.sleep(500);

System.out.println("Current Thread: "

+ Thread.currentThread().getName());

}

catch(Exception ex)

{

System.out.println("Exception has"+

" been caught"+ ex);

}

System.out.println(i);

}

}

}

classGFG

{

publicstaticvoidmain (String[] args)

{

// creating two threads

ThreadJoining t1 = newThreadJoining();

ThreadJoining t2 = newThreadJoining();

ThreadJoining t3 = newThreadJoining();

// thread t1 starts

t1.start();

// starts second thread after when

// first thread t1 is died.

try

{

System.out.println("Current Thread: "

+ Thread.currentThread().getName());

t1.join();

}

catch(Exception ex)

{

System.out.println("Exception has "+

"been caught"+ ex);

}

// t2 starts

t2.start();

// starts t3 after when thread t2 is died.

try

{

System.out.println("Current Thread: "

+ Thread.currentThread().getName());

t2.join();

}

catch(Exception ex)

{

System.out.println("Exception has been"+

" caught"+ ex);

}

t3.start();

}

}

output:

Current Thread: main
Current Thread: Thread-0
0
Current Thread: Thread-0
1
Current Thread: main
Current Thread: Thread-1
0
Current Thread: Thread-1
1
Current Thread: Thread-2
0
Current Thread: Thread-2
1

In the above example we can see clearly second thread t2 starts after first thread t1 is died and t3 will start its execution after second thread t2 is died.

This article is contributed by Nitsdheerendra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.